The highly venomous snake had spent the journey just centimetres from where her child was sitting.
Kathy Pablo of Weipa in far north Queensland said she was “grateful” the family was safe and well.
The pictures were posed on a community group’s Facebook page in Toowoomba and shows the snake poking its head out from beneath her son’s car seat.
“I left my window an inch open and this fellow found his way into the car and went for a ride 20kms into town under the car seat,” Ms Pablo said in the post. “Only to pop its head out while sitting in the car.
“It was underneath the seat where my little boy was sitting.
“Please be careful, the drought has made all the wildlife come closer in search of food and water. Feeling very grateful we are all safe and well.”
Ms Pablo told 7 News that the family, and the snake, all survived the ordeal without a scratch.
Eastern browns can grow up to 2m in length and are found throughout eastern Australia including populated parts of South Australia.
The species has been responsible for numerous deaths and has a strong neurotoxin in its venom. If bitten, people should not wash off the venom as medics can use it to identify the type of snake and the anti venom that should be administered.
The advice is to call triple-0 and apply a pressure immobilisation bandage and keep the person calm and as still as possible until medical help arrives. CPR may be needed.
Earlier this month, a Queensland snake catcher shared photos of the biggest deadly eastern brown snake he had ever seen.
He said the monster serpent was more than 2m long and as “thick and healthy” as a coke can.
“Biggest eastern brown snake I have ever seen,” Stuart McKenzie, who runs Queensland snake wrangling business Sunshine Coast Snake Catchers 24/7 wrote on Facebook.
Mr McKenzie said his new business manager, Donnie, had managed to capture the “heavy” snake from a home the Sunshine Coast yesterday.
“We predict this snake to be well over two meters and nearly as thick as a coke can! It was heavy to hold as well! So thick and so healthy. Wow! What a snake and what a day! Yeowww!”
News.com.au has contacted Ms Pablo for comment.